1. Technical Field of Invention
This invention relates to aerodynamic lifting surfaces; in particular, surfaces having a serrated-planform shape.
2. Description of the Related Art
High-angle-of-attack aerodynamics is of great importance to modern peace-keeping aircraft where the ability to fly at stall and post-stall angles-of-attack can mean the difference between life and death. To efficiently fly an aircraft at stall and post-stall conditions, the aircraft should be capable of achieving exceedingly high lift and having exceptional handling capabilities over an extremely wide range of flight conditions.
Fighter aircraft often employ thin, highly swept wings with low aspect ratios to minimize wave drag at supersonic cruise and to enhance maneuverability at all speeds. The prototypical wing designs include variations of arrow-, delta-, and diamond-shaped planforms. On all of these wings, at positive angles-of-attack, the boundary layer on the pressure surface moves outward and separates as it goes around the leading edge. This process forms a free-shear layer that curves upward and then inboard, rolling into a wing vortex on the suction surface of the wing. The wing vortex contributes substantially to the total lifting force on the wing. The total lift coefficient for the wing is conveniently split into a sum of two distinct contributions, a first contribution that accounts for the ordinary potential-flow lift, and a second contribution, known as the vortex-induced lift. Out of context, the term "vortex-induced lift" can be misleading, since all lift is due to vorticity in the flow. Here we use the term "vortex-induced lift" to refer to the extra lift produced by the presence of one or more coherent vortex structures over the wing.
Various forebody devices, such as leading-edge extensions, strakes, and canards provide additional vortex-shedding surfaces that are known to be useful for further increasing the lift and improving aerodynamic characteristics. Each forebody typically produces one or more strong forebody vortices that often persist over a primary lifting surface or wing and contribute to the vortex-induced lift. As with the wing vortices, the forebody vortices energize the boundary layer on the suction surface of the primary wing, thus helping to keep the primary-wing boundary layer attached to the primary wing at higher angles-of-attack. The maintenance of an attached boundary layer over the primary wing, greatly increases the lift and the lift-to-drag efficiency of the aircraft.
As an example, Skow U.S. Pat. No. 5,249,762 (1993)! discloses strakes mounted forward and above the plane of the primary wing. The strakes reduce buffet on the wings and result in a more positive pitching moment and therefore better longitudinal stability at high angles of attack. These features allow for landings at higher angles-of-attack, with the resultant higher lift coefficient and decreased landing speed. The intended embodiment of the invention is for use on a T-38 aircraft. On this aircraft, the strakes are mounted on the engine nacelles, just forward of the wing leading edge, and at an elevation substantially above the plane of the wing. On the T-38, the patent claims that the invention performs better than leading-edge extensions.
In general, the vortex-induced lift acts only as long as the relevant vortex structures remain coherent and positioned near the wing surface. Unfortunately, at large angles-of-attack, both the wing and forebody vortices become unstable and burst into largely incoherent masses of turbulence. The further aft the breakdown occurs, the longer the extent of the region of vortex-induced lift. Lamar "Nonlinear lift control at high speed and high angle of attack using vortex flow technology," in AGARD Report 740 (1982), available from NTIS, 5285 Port Royal Rd, Springfield, Va. 22161! discusses a variety of methods used to delay the onset of vortex bursting. Lamar (1982) states that three particularly promising innovations for delaying vortex breakdown are: the fluid strake (a jet sheet formed by blowing through a series of in-line orifices on the fuselage ahead of the wing); spanwise blowing over the main wing; and vortex pumping through the use of suction on the primary wing. Note that he does not mention the use of additional lifting surfaces in his list of promising innovations.
Computations by Kern AIAA Paper 92-0411 (1992), available from AIAA, 370 L'Enfant Promenade, S.W., Washington, D.C. 20024! suggest that minor changes in the geometry of a strake-wing junction can vastly alter the interaction of the strake vortex with the wing vortex. These geometry changes can be used to vary the vortex-breakdown position. Walters and Kern U.S. Pat. No. 5,282,591 (1994)! use these ideas to develop active vortex control for a high-performance wing. The invention comprises two slidably adjustable panels that can be arranged in various configurations during flight. The panels, when not in use, are retracted and stowed, thus resulting in a first strake/wing junction configuration that produces a strake vortex and a wing vortex. A second configuration produces a continuous transition from the strake to the wing. The second configuration results in a combined strong vortex that bursts closer to the apex of the wing, thus reducing the lift on the wing. A third configuration that employs a straight-line transition from the strake to the wing has an additional abrupt junction that produces a fillet vortex, which is weaker than the strake and wing vortices. The burst location of the vortices is further downstream compared to the first configuration. Finally, a fourth configuration provides a diamond-shaped fillet that adds yet another junction. The fillet vortex produced in this configuration is weaker than that of the third configuration. The vortex burst point is further delayed.
Because the highly nonlinear nature of vortex flows makes the behavior of the forebody vortices and the wing vortices very difficult to predict, another very useful approach for delaying vortex breakdown and dramatically increasing vortex-induced lift over a wide range of angles-of-attack has been overlooked by all prior innovators.